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Old 08-13-2014, 05:48 PM   #1
Ramanonos OP
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Spark plug skid marks

Been getting an '89 R100GS into fighting shape. After a whole lot of valve and carb tweaking, I finally her running nicely. Today I peeked at both spark plugs to see condition (new, but inside during carb tweaking). The right side looked great; however the left side showed a black streak down one side. See pic.
I'm wondering if this might be indicative of a bigger problem with piston rings or something else. I've never seen a plug streaked on one side like that.
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Old 08-13-2014, 07:19 PM   #2
Tobz
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Try checking the positioning the plug to see if the oil mark is on the intake or exhaust side. My 89 GS did the same thing on the intake side of the right cylinder. First thing I tried was moving the crankcase ventilation to rule out blowby, or the oil source being the airbox. It wasn't, so I disassembled the heads and the valve guides were shot.
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Old 08-13-2014, 09:10 PM   #3
rustygardhouse
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I use a sharpy to mark the insulator on the the ground strap side so I know the orientation of the plug in the cylinder. I try to keep the strap pointed towards the intake valve.

I see that black streak on intake side of the electrode when it cools off & I'm using the choke longer. Knew it was a rich condition but always thought it was from the choke & never thought about worn guides...
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Old 08-14-2014, 07:34 AM   #4
Roach Carver
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I get the same thing on my guzzi. fresh head work. Did I get a bad head job?
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Old 08-14-2014, 08:12 AM   #5
Tobz
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Quote:
Originally Posted by Roach Carver View Post
I get the same thing on my guzzi. fresh head work. Did I get a bad head job?
Some Guzzi are like an airhead in that they don't use valve seals, so proper guide clearance is critical. Maybe try a fresh plug and ride it around without doing any compression braking and see if you get the same symptoms. Oil is more likely to be drawn through the guides during deceleration if they are loose. Could also be blowby or a gasket. The extra cylinder pressure from fresh valves can somtimes create sealing issues with old rings.
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Old 08-14-2014, 09:30 AM   #6
Roach Carver
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thanks, full disclosure is that it has new nigusil cylinders, rings and pistons. Had the heads were done locally by a machine shop that I have no experience with.

oh yeah and new gaskets and heads/cylinders have been re torqued. Sorry to the OP for the thread jack.
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Old 08-14-2014, 03:57 PM   #7
RVFlyer
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The OP's plug looks like my left one usually does - I thought I saw something on another thread a few months back that this is not uncommon on left side plugs if the bike has been on the side stand and some oil draws in past the rings or guides as the engine cools. The fact it is specific to one side of the plug makes me think the side stand theory may be correct. I don't notice any other symptoms of something being wrong on the left cylinder, as far as I know.
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Old 08-14-2014, 07:16 PM   #8
bmwrench
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This looks like a very fresh plug, with an indication of rich running on the inside of the shell. If you've been doing a lot of tweaking on the carbs without a run on the road, this will probably disappear once you do.
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Old 08-15-2014, 08:19 PM   #9
Ramanonos OP
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Thanks. My carbs are tuned with good mix and valves are adjusted. I just swapped in a new plug and will see if it happens again.
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